Printed circuit board and electronic component

ABSTRACT

A printed circuit board according to an embodiment of the present invention includes a base film having an insulating property and a conductive pattern disposed on at least one surface of the base film. The conductive pattern includes a copper particle bond layer which is fixed to the base film, and a lightness L* of a conductive pattern non-formed region of the base film is 60 or less. The base film may include a modified layer on one surface side thereof.

TECHNICAL FIELD

The present invention relates to a printed circuit board and anelectronic component.

The present application is based upon and claims the benefit of priorityfrom Japanese Patent Application No. 2015-160592, filed Aug. 17, 2015,the entire contents of which are incorporated herein by reference.

BACKGROUND ART

There is a known printed circuit board which includes a base film and aconductive pattern disposed on one surface of the base film. Such aprinted circuit board is fabricated, for example, by forming a seedlayer with a thickness of 1 μm or less by a sputtering method on onesurface of a base film, forming a metal plating layer by electroplatingon one surface of the seed layer to obtain a metal layer, and etchingthe metal layer into a desired pattern (refer to Japanese UnexaminedPatent Application Publication No. 9-136378).

Usually after formation of circuits, the printed circuit board isinspected to determine the presence or absence of circuit defects withan automated optical inspection system, and is then shipped as aproduct. Furthermore, in the inspection with the automated opticalinspection system, the printed circuit board is irradiated with light,and the presence or absence of circuit defects is determined on thebasis of the contrast of reflected light.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 9-136378

SUMMARY OF INVENTION

A printed circuit board according to an embodiment of the presentinvention includes a base film having an insulating property and aconductive pattern disposed on at least one surface of the base film.The conductive pattern includes a copper particle bond layer which isfixed to the base film, and a lightness L* of a conductive patternnon-formed region of the base film is 60 or less.

Furthermore, an electronic component according to another embodiment ofthe present invention includes the printed circuit board and an elementmounted on the printed circuit board.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of a printed circuit boardaccording to an embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view showing a printed circuitboard according to an embodiment which is different from the printedcircuit board shown in FIG. 1.

FIG. 3 is a schematic cross-sectional view showing a printed circuitboard according to an embodiment which is different from the printedcircuit board shown in FIG. 1 or 2.

FIG. 4A is a schematic cross-sectional view showing a coating filmformation step of a method for manufacturing a printed circuit boardaccording to the present invention.

FIG. 4B is a schematic cross-sectional view showing a copper particlebond layer formation step of the method for manufacturing a printedcircuit board according to the present invention.

FIG. 4C is a schematic cross-sectional view showing a metal platinglayer formation step of the method for manufacturing a printed circuitboard according to the present invention.

FIG. 4D is a schematic cross-sectional view showing a metal platinglayer formation step according to an embodiment which is different fromthe metal plating layer formation step shown in FIG. 4C.

DESCRIPTION OF EMBODIMENTS

[Problems to be Solved by the Present Disclosure]

In the existing printed circuit board described above, it is difficultto sufficiently obtain a contrast between the circuit surface and thesurface of the base film exposed between circuits. When the contrastbetween the circuit surface and the surface of the base film isinsufficient, it becomes difficult to accurately recognize circuits withan automated optical inspection system, and as a result, the inspectionerror rate in determination of circuit defects increases.

The present invention has been made under these circumstances. It is anobject of the invention to provide a printed circuit board which canreduce the inspection error rate in determination of circuit defects andan electronic component which can reduce the rate of occurrence ofcircuit defects.

[Advantageous Effects of the Present Disclosure]

A printed circuit board according to an embodiment of the presentinvention can reduce the inspection error rate in determination ofcircuit defects. Furthermore, an electronic component according to thepresent invention can reduce the rate of occurrence of circuit defects.

[Description of Embodiments of the Present Invention]

First, the embodiments of the present invention will be described belowone by one.

A printed circuit board according to an embodiment of the presentinvention includes a base film having an insulating property and aconductive pattern disposed on at least one surface of the base film.The conductive pattern includes a copper particle bond layer which isfixed to the base film, and a lightness L* of a conductive patternnon-formed region of the base film is 60 or less.

In the printed circuit board, since the lightness L* of the conductivepattern non-formed region of the base film is equal to or less than theupper limit described above, it is possible to easily increase thedifference between the lightness L* of one surface of the conductivepattern and the lightness L* of the conductive pattern non-formed regionof the base film. Accordingly, in the printed circuit board, bysufficiently ensuring the contrast between the one surface of theconductive pattern and the conductive pattern non-formed region of thebase film, circuit defects can be easily detected with high accuracy bylight emitted from a circuit defect inspection system. Therefore, theprinted circuit board can reduce the inspection error rate indetermination of circuit defects.

The base film may include a modified layer on one surface side thereof.When the base film includes the modified layer on one surface sidethereof, the lightness L* of the conductive pattern non-formed region ofthe base film can be easily and reliably adjusted to be in the rangedescribed above by the modified layer. Furthermore, by adjusting thelightness L* of the conductive pattern non-formed region of the basefilm to be in the range described above by the modified layer, thematerial properties of components constituting the base film can beeasily maintained in a region other than the modified layer. As aresult, it is possible to suppress degradation in the quality of thebase film.

Preferably, the base film does not substantially contain a pigment. Whenthe base film does not substantially contain a pigment, it is possibleto reduce the inspection error rate in determination of circuit defectswhile more reliably maintaining the material properties of the basefilm.

Preferably, a main component of the base film is a polyimide. When themain component of the base film is a polyimide, while suppressingdegradation in the quality of the base film, the lightness L* of theconductive pattern non-formed region of the base film can be easilyadjusted to be in the range described above.

Preferably, a chromaticity b* of the conductive pattern non-formedregion of the base film is 60 or less. When the chromaticity b* of theconductive pattern non-formed region of the base film is in the rangedescribed above, it is possible to further reduce the inspection errorrate in determination of circuit defects.

The absolute value of a difference between the lightness L* of onesurface of the conductive pattern and the lightness L* of the conductivepattern non-formed region of the base film is preferably 10 or more.When the absolute value of the difference between the lightness L* ofone surface of the conductive pattern and the lightness L* of theconductive pattern non-formed region of the base film is equal to ormore than the lower limit, the contrast between the conductive patternnon-formed region of the base film and the one surface of the conductivepattern can be promoted, and it is possible to further reduce theinspection error rate in determination of circuit defects.

An electronic component according to an embodiment of the presentinvention includes the printed circuit board and an element mounted onthe printed circuit board.

Since the electronic component includes the printed circuit board whichcan reduce the inspection error rate in determination of circuitdefects, it is possible to reduce the rate of occurrence of circuitdefects.

Note that, in the present invention, the terms “lightness” and“chromaticity” mean the lightness and chromaticity defined by the L*a*b*and refer to values conforming to JIS-Z8781-4 (2013). The term “copperparticle bond layer” refers to a layer including a structure in which aplurality of copper particles adhere and bond together. The expression“does not substantially contain a pigment” refers to the fact that apigment is not positively incorporated, except for inevitable inclusion,and for example, that the pigment content is 1% by mass or less,preferably 0.1% by mass or less, and more preferably 0.01% by mass orless. The term “main component” refers to a component whose content isthe largest, for example, a component with a content of 50% by mass ormore, preferably 80% by mass or more.

[Detailed Description of Embodiments of the Present Invention]

Printed circuit boards according to the embodiments of the presentinvention will be described below with reference to the drawingsappropriately.

[First Embodiment]

<Printed Circuit Board>

A printed circuit board 1 shown in FIG. 1 includes a base film 2 and aconductive pattern 3 disposed on one surface of the base film 2. Theprinted circuit board 1 shown in FIG. 1 is a flexible printed circuitboard having flexibility.

(Base Film)

The base film 2 has an insulating property and flexibility. The basefilm 2 is a single resin film containing a synthetic resin as a maincomponent. Examples of the main component of the base film 2 includesynthetic resins, such as a polyimide, polyethylene terephthalate, afluororesin, and a liquid crystal polymer. Among these, preferred is apolyimide which has excellent insulating property, flexibility, heatresistance, and the like. Furthermore, since the polyimide has high heatresistance, as will be described later, even in the case where onesurface side of the base film 2 is modified, it is possible to preventdegradation in quality, such as heat deformation, of the base film 2.

The lower limit of the average thickness of the base film 2 ispreferably 5 μm, more preferably 12 μM, and still more preferably 25 μm.On the other hand, the upper limit of the average thickness of the basefilm 2 is preferably 2 mm, more preferably 1.6 mm, still more preferably500 μm, and particularly preferably 200 μm. When the average thicknessof the base film 2 is less than the lower limit, there is a concern thatthe insulating property and mechanical strength may become insufficient.Contrarily, when the average thickness of the base film 2 exceeds theupper limit, there is a concern that the requirement for reduction inthickness may not be satisfied. On the other hand, when the averagethickness of the base film 2 is within the range described above,reduction in thickness can be promoted while suppressing degradations inthe insulating property and mechanical strength of the base film 2.Furthermore, in the printed circuit board 1, even when the averagethickness of the base film 2 is decreased in the range described above,the lightness L* of a conductive pattern non-formed region X(hereinafter, may also be referred to as the “non-formed region X”) canbe sufficiently decreased. Note that the term “average thickness” refersto an average of the values measured at arbitrary ten points.

The base film 2 includes a modified layer 2 a on one surface sidethereof. The modified layer 2 a adjusts the lightness L* andchromaticities a* and b* of the non-formed region X. The modified layer2 a is formed by modifying the one surface side of the base film 2 whena copper particle bond layer 3 a, which will be described later, isfixed to one surface of the base film 2. The composition of the modifiedlayer 2 a varies from that of the other region (non-modified layer 2 b)of the base film 2. The base film 2 has a two-layered structureincluding the modified layer 2 a and the non-modified layer 2 b. In theprinted circuit board 1, since the base film 2 includes the modifiedlayer 2 a on one surface side thereof, the lightness L* andchromaticities a* and b* of one surface of the base film 2 can be easilyand reliably adjusted by the modified layer 2 a. Furthermore, in theprinted circuit board 1, since the lightness L* and chromaticities a*and b* of one surface of the base film 2 are adjusted by the modifiedlayer 2 a, the material properties of components constituting the basefilm 2 can be easily maintained in the non-modified layer 2 b. As aresult, in the printed circuit board 1, it is possible to suppressdegradation in quality of the base film 2.

The modified layer 2 a is formed in the entire region of the base film 2when viewed in plan. Furthermore, the modified layer 2 a includes afixing surface for the copper particle bond layer 3 a (one surface ofthe base film 2) serving as an outermost surface and has a certainthickness. The lower limit of the average thickness of the modifiedlayer 2 a is preferably 5 nm, and more preferably 10 nm. On the otherhand, the upper limit of the average thickness of the modified layer 2 ais preferably 100 nm, and more preferably 50 nm. When the averagethickness of the modified layer 2 a is less than the lower limit, thereis a concern that the lightness L* and chromaticities a* and b* of theone surface of the base film 2 may not be sufficiently adjusted.Contrarily, when the average thickness of the modified layer 2 a exceedsthe upper limit, there is a concern that the thickness of thenon-modified layer 2 b which is important for maintaining the quality ofthe base film 2 may be decreased unnecessarily.

Preferably, one surface of the base film 2 is subjected to a hydrophilictreatment, and then the modified layer 2 a is formed. That is,preferably, one surface of the base film 2 is subjected to a hydrophilictreatment, and then the copper particle bond layer 3 a is fixed to theone surface of the base film 2. As the hydrophilic treatment, forexample, a plasma treatment in which the fixing surface is madehydrophilic by irradiation with plasma, or alkaline treatment in whichthe fixing surface is made hydrophilic by using an alkaline solution canbe used. By subjecting the one surface of the base film 2 to ahydrophilic treatment, surface tension of an ink with respect to the onesurface decreases, and therefore, the ink can be uniformly applied tothe one surface. In particular, a plasma treatment is preferable as thehydrophilic treatment. In the printed circuit board 1, by performing aplasma treatment as the hydrophilic treatment and further by forming themodified layer 2 a, the lightness L* and chromaticities a* and b* of theone surface of the base film 2 can be more appropriately adjusted.Furthermore, in the printed circuit board 1, it is also preferable toperform a blast treatment in addition to the plasma treatment. As theblast treatment, in particular, a wet blast treatment which is performedwith inorganic particles being dispersed in a liquid is more preferable.

Preferably, the base film 2 does not substantially contain a pigment. Inthe printed circuit board 1, when the base film 2 does not substantiallycontain a pigment, it is possible to reduce the inspection error rate indetermination of circuit defects while more reliably maintaining thematerial properties of the base film 2.

The upper limit of the lightness L* of the non-formed region X of thebase film 2 is 60, preferably 55, and more preferably 50. When thelightness L* of the non-formed region X of the base film 2 exceeds theupper limit, there is a concern that it may not be possible tosufficiently obtain a contrast between the one surface of the conductivepattern 3 and the non-formed region X of the base film 2. The lowerlimit of the lightness L* of the non-formed region X of the base film 2is not particularly limited, but can be, for example, 30.

The upper limit of the chromaticity b* of the non-formed region X of thebase film 2 is preferably 60, more preferably 45, and still morepreferably 37. When the chromaticity b* of the non-formed region X ofthe base film 2 exceeds the upper limit, there is a concern that it maynot be possible to sufficiently obtain a contrast between the non-formedregion X of the base film 2 and the one surface of the conductivepattern 3. The lower limit of the chromaticity b* of the non-formedregion X of the base film 2 is not particularly limited, but can be, forexample, 20.

The upper limit of the chromaticity a* of the non-formed region X of thebase film 2 is preferably 22, and more preferably 20. When thechromaticity a* of the non-formed region X of the base film 2 exceedsthe upper limit, there is a concern that it may not be possible tosufficiently obtain a contrast between the non-formed region X of thebase film 2 and the one surface of the conductive pattern 3. The lowerlimit of the chromaticity a* of the non-formed region X of the base film2 is not particularly limited, but can be, for example, 10.

The upper limit of the external transmittance for a wavelength of 500 nmin the non-formed region X of the base film 2 is preferably 15%, morepreferably 12%, and still more preferably 10%. When the externaltransmittance exceeds the upper limit, there is a concern that lightemitted from a circuit defect inspection system may become easilytransmitted through the base film 2, resulting in a decrease ininspection accuracy. On the other hand, since the external transmittanceis preferably as low as possible, the lower limit of the externaltransmittance is not particularly limited, but can be, for example, 1%.Note that the external transmittance can be adjusted by forming themodified layer 2 a in the base film 2, and specifically, can be adjustedby the firing temperature, firing time, and the like when the copperparticle bond layer 3 a is formed on the one surface of the base film 2.The term “external transmittance” refers to a transmittance which takesinto consideration surface reflection, and specifically means the ratio(I/I₀) of transmitted light intensity (I) to incident light intensity(I₀). Furthermore, the term “light intensity” refers to incident lightflux per unit area.

(Conductive Pattern)

The conductive pattern 3 includes a copper particle bond layer 3 a fixedto the base film 2. In particular, in this embodiment, the conductivepattern 3 consists of only the copper particle bond layer 3 a. Theconductive pattern 3 is obtained by forming a copper particle bond layer3 a on the entire one surface of the base film 2, followed by patterningthe copper particle bond layer 3 a. As the formation method for thecopper particle bond layer 3 a, as will be described later, a method inwhich an ink containing copper particles is applied to one surface ofthe base film 2, followed by firing may be used. In this case, thecopper particle bond layer 3 a is formed as a copper particle sinteredlayer. As the patterning method of forming the conductive pattern 3, forexample, a method (subtractive method) in which the copper particle bondlayer 3 a formed on the entire one surface of the base film 2 is maskedwith a resist pattern or the like and etching is performed can be used.Since the conductive pattern 3 includes such a copper particle bondlayer 3 a, conduction can be improved while suppressing manufacturingcosts. Furthermore, since the conductive pattern 3 includes such acopper particle bond layer 3 a, a modified layer 2 a can be formed onthe one surface side of the base film 2.

The lower limit of the mean size of copper particles constituting thecopper particle bond layer 3 a is preferably 1 nm, more preferably 10nm, and still more preferably 30 nm. On the other hand, the upper limitof the mean particle size of the copper particles is preferably 500 nm,more preferably 300 nm, and still more preferably 100 nm. When the meanparticle size of the copper particles is less than the lower limit,there is a concern that the dispersibility and stability of copperparticles in the ink used during formation of the copper particle bondlayer 3 a may be decreased. Contrarily, when the mean particle size ofthe copper particles exceeds the upper limit, there is a concern thatthe copper particles may be likely to be precipitated and that thedensity of the copper particles may become non-uniform when the ink isapplied. Note that the term “mean particle size” refers to a meanparticle size represented by the volume median diameter D50 of theparticle size distribution of copper particles in a dispersion liquid.

The lower limit of the average thickness of the copper particle bondlayer 3 a is preferably 10 nm, more preferably 50 nm, and still morepreferably 100 nm. On the other hand, the upper limit of the averagethickness of the copper particle bond layer 3 a is preferably 1 μm, morepreferably 700 nm, and still more preferably 500 nm. When the averagethickness of the copper particle bond layer 3 a is less than the lowerlimit, there is a concern that breaks may occur in the copper particlebond layer 3 a when viewed in plan, and it may become difficult to formthe modified layer 2 a over the entire region. Contrarily, when theaverage thickness of the copper particle bond layer 3 a exceeds theupper limit, there is a concern that it may take time to remove thecopper particle bond layer 3 a between conductive patterns 3 when asemi-additive method is used for interconnection formation, andproductivity may be decreased.

The lower limit of the separation strength between the base film 2 andthe copper particle bond layer 3 a is preferably 1 N/cm, more preferably1.5 N/cm, still more preferably 2 N/cm, and particularly preferably 5N/cm. By setting the separation strength to be equal to or more than thelower limit, it is possible to fabricate a printed circuit board havinghigh electrical connection reliability. On the other hand, the upperlimit of the separation strength is not particularly limited, but canbe, for example, about 20 N/cm. The separation strength can becontrolled, for example, by the amount of copper particles fixed to thebase film 2, the size of copper particles in an ink, which will bedescribed later, the firing temperature and firing time during firing acoating film, which will be described later, and the like.

The lower limit of the lightness L* of the one surface of the conductivepattern 3 is preferably 40, and more preferably 50. When the lightnessL* of the one surface of the conductive pattern 3 is less than the lowerlimit, there is a concern that it may not be possible to sufficientlyobtain a contrast between the non-formed region X of the base film 2 andthe one surface of the conductive pattern 3. The upper limit of thelightness L* of the one surface of the conductive pattern 3 is notparticularly limited, but can be, for example, 90.

The lower limit of the absolute value of the difference between thelightness L* of the one surface of the conductive pattern 3 and thelightness L* of the non-formed region X of the base film 2 is preferably10, more preferably 15, and still more preferably 20. When the absolutevalue of the difference in the lightness L* is less than the lowerlimit, there is a concern that it may not possible to sufficientlyobtain a contrast between the non-formed region X of the base film 2 andthe one surface of the conductive pattern 3 and it may not be possibleto sufficiently improve inspection accuracy for circuit defects. Theupper limit of the absolute value of the difference in the lightness L*is not particularly limited, but can be, for example, 40.

<Advantages>

In the printed circuit board 1, since the lightness L* of the conductivepattern non-formed region of the base film 2 is equal to or less thanthe upper limit described above, it is possible to easily increase thedifference between the lightness L* of one surface of the conductivepattern 3 and the lightness L* of the conductive pattern non-formedregion of the base film 2. Accordingly, in the printed circuit board 1,by sufficiently ensuring the contrast between the one surface of theconductive pattern 3 and the conductive pattern non-formed region of thebase film 2, circuit defects can be easily detected with high accuracyby light emitted from a circuit defect inspection system. Therefore, theprinted circuit board 1 can reduce the inspection error rate indetermination of circuit defects.

[Second Embodiment]

<Printed Circuit Board>

A printed circuit board 11 shown in FIG. 2 is a flexible printed circuitboard having flexibility. The printed circuit board 11 shown in FIG. 2includes a base film 2 and a conductive pattern 12 disposed on onesurface of the base film 2. The printed circuit board 11 shown in FIG. 2is the same as the printed circuit board 1 shown in FIG. 1 except thatthe conductive pattern 12 includes a metal plating layer 12 a on theouter surface of the copper particle bond layer 3 a of the printedcircuit board 1 shown in FIG. 1. Furthermore, as the patterning methodof forming the conductive pattern 12, for example, a subtractive methodcan be used, as in the printed circuit board 1 shown in FIG. 1. The basefilm 2 and the copper particle bond layer 3 a in the printed circuitboard 11 are the same as those in the printed circuit board 1 shown inFIG. 1. Accordingly, they are denoted by the same reference signs, and adescription thereof is omitted.

(Metal Plating Layer)

The metal plating layer 12 a is formed by filling voids of the copperparticle bond layer 3 a with a plating metal and depositing the platingmetal on one surface of the copper particle bond layer 3 a. Furthermore,all the voids of the copper particle bond layer 3 a are filled with theplating metal. In the printed circuit board 11, since the voids of thecopper particle bond layer 3 a are filled with the plating metal, it ispossible to suppress the void portions of the copper particle bond layer3 a from acting as starting points for breakage to cause separation ofthe conductive pattern 12 from the base film 2. Furthermore, in theprinted circuit board 11, since the voids of the copper particle bondlayer 3 a are filled with the plating metal, by performing a heattreatment after filling with the plating metal, modification of the basefilm 2 can be further promoted.

The plating method for forming the metal plating layer 12 a is notparticularly limited, and may be electroless plating or electroplating.It is preferable to use electroless plating by which the voids betweenthe copper particles constituting the copper particle bond layer 3 a canbe more appropriately filled, and the effect of modifying the base film2 is easily improved by performing a heat treatment after filling withthe plating metal.

As the metal constituting the metal plating layer 12 a, highlyconductive copper, nickel, silver, or the like can be used. Inconsideration of adhesion with the copper particles, use of copper ornickel is preferable.

The lower limit of the average thickness of the metal plating layer 12 ais preferably 50 nm, more preferably 100 nm, and still more preferably200 nm. On the other hand, the upper limit of the average thickness ofthe metal plating layer 12 a is preferably 2 μm, more preferably 1.5 μm,and still more preferably 1 μm. When the average thickness of the metalplating layer 12 a is less than the lower limit, there is a concern thatthe voids of the copper particle bond layer 3 a may not be sufficientlyfilled with a plating metal. Contrarily, when the average thickness ofthe metal plating layer 12 a exceeds the upper limit, for example, inthe case where the metal plating layer 12 a is formed by electrolessplating, there is a concern that a long time may be required for theelectroless plating, resulting in a decrease in productivity.

In this embodiment, the metal plating layer 12 a is formed by fillingvoids of the copper particle bond layer 3 a with a plating metal anddepositing the plating metal on one surface of the copper particle bondlayer 3 a. However, as long as the voids of the copper particle bondlayer 3 a are filled with the plating metal, the metal plating layer 12a is not necessarily deposited to the one surface of the copper particlebond layer 3 a.

<Advantages>

In the printed circuit board 11, since the metal plating layer 12 a isdisposed on the outer surface of the copper particle bond layer 3 a, byperforming a heat treatment after filling with the plating metal,modification of the base film 2 can be promoted to facilitate adjustmentof the lightness L* and chromaticities a* and b* of the base film 2.

[Third Embodiment]

<Printed Circuit Board>

A printed circuit board 21 shown in FIG. 3 is a flexible printed circuitboard having flexibility. The printed circuit board 21 shown in FIG. 3includes a base film 2 and a conductive pattern 23 disposed on onesurface of the base film 2. The printed circuit board 21 shown in FIG. 3is the same as the printed circuit board 1 shown in FIG. 1 except thatthe conductive pattern 23 includes a metal plating layer on the outersurface of the copper particle bond layer 3 a of the printed circuitboard 1 shown in FIG. 1. Furthermore, as the patterning method offorming the conductive pattern 23, for example, a subtractive method canbe used, as in the printed circuit board 1 shown in FIG. 1. The basefilm 2 and the copper particle bond layer 3 a in the printed circuitboard 21 are the same as those in the printed circuit board 1 shown inFIG. 1. Accordingly, they are denoted by the same reference signs, and adescription thereof is omitted.

(Metal Plating Layer)

The metal plating layer includes a first plating layer 12 a and a secondplating layer 22 a. The first plating layer 12 a has the same structureas the metal plating layer 12 a shown in FIG. 2.

(Second Plating Layer)

The second plating layer 22 a is disposed on one surface of the firstplating layer 12 a. The plating method for forming the second platinglayer 22 a is not particularly limited, and may be electroless platingor electroplating. It is preferable to use electroplating by which thethickness can be adjusted easily and accurately and the second platinglayer 22 a can be formed in a relatively short period of time.

As the metal constituting the second plating layer 22 a, for example,highly conductive copper, nickel, silver, or the like may be used.

The average thickness of the second plating layer 22 a is set dependingon what type of printed circuit is fabricated and is not particularlylimited. For example, the average thickness can be 1 to 100 μm.

<Advantages>

In the printed circuit board 21, since the metal plating layer includesthe first plating layer 12 a and the second plating layer 22 a, thethickness of the conductive pattern 23 can be adjusted easily andreliably.

<Method for Manufacturing Printed Circuit Board>

Methods for manufacturing the printed circuit boards 1, 11, and 21 willbe described below with reference to FIGS. 4A to 4D.

First, a method for manufacturing the printed circuit board 1 will bedescribed with reference to FIGS. 4A and 4B. The method formanufacturing the printed circuit board 1 includes a step of forming acoating film 42 by applying an ink containing copper particles 41 to onesurface of a base film 2, a step of forming a copper particle bond layer3 a (copper particle sintered layer) by firing the coating film 42, anda step of patterning the copper particle bond layer 3 a.

(Coating Film Formation Step)

In the coating film formation step, as shown in FIG. 4A, a coating film42 is formed by applying an ink containing copper particles 41 to onesurface of a base film 2, followed by, for example, drying. The coatingfilm 42 may contain a dispersion medium and the like of the ink.

(Copper Particles)

Copper particles 41 to be dispersed in the ink can be produced by ahigh-temperature treatment method, liquid-phase reduction method,gas-phase method, or the like. In particular, in the case where aliquid-phase reduction method is used, manufacturing costs can befurther reduced, and the particle size of the copper particles 41 can beeasily made uniform by stirring in the aqueous solution, or the like.

In order to produce copper particles 41 by the liquid-phase reductionmethod, for example, a water-soluble copper compound, which is a sourcefor copper ions constituting the copper particles 41, and a dispersingagent are dissolved in water, and by adding a reducing agent thereinto,copper ions are subjected to reduction for a certain period of time. Thecopper particles 41 produced by the liquid-phase reduction method arespherical or granular in shape and uniform, and moreover, fine particlescan be obtained. Examples of the water-soluble copper compound which isa source for copper ions include copper (II) nitrate (Cu(NO₃)₂), copper(II) sulfate pentahydrate (CuSO₄·5H₂O), and the like.

As the reducing agent, various reducing agents that are able to reduceand precipitate copper ions in a liquid-phase (aqueous solution)reaction system can be used. Examples of the reducing agent includesodium borohydride, sodium hypophosphite, hydrazine, transition metalions such as trivalent titanium ions and divalent cobalt ions, ascorbicacid, reducing saccharides such as glucose and fructose, polyhydricalcohols such as ethylene glycol and glycerol, and the like. Amongthese, trivalent titanium ions are preferable as the reducing agent.Note that the liquid-phase reduction method in which trivalent titaniumions are used as the reducing agent is referred to as a “titanium redoxprocess”. In the titanium redox process, copper ions are reduced byoxidation-reduction when trivalent titanium ions are oxidized totetravalent ions to precipitate copper particles 41. The copperparticles 41 obtained by the titanium redox process have a smallparticle size and are uniform. Therefore, the copper particles 41 arepacked at a higher density, and the coating film 42 can be made denser.

The particle size of the copper particles 41 can be controlled byadjusting the types and mixing ratios of the copper compound, dispersingagent, and reducing agent, and by adjusting the stirring rate,temperature, time, pH, and the like during reduction of the coppercompound. The lower limit of the pH of the reaction system is preferably7, and the upper limit of the pH of the reaction system is preferably13. By setting the pH of the reaction system to be in the rangedescribed above, it is possible to obtain copper particles 41 having avery small particle size. At this time, by using a pH adjuster, the pHof the reaction system can be easily adjusted to be in the rangedescribed above. As the pH adjuster, a common acid or alkali, such ashydrochloric acid, sulfuric acid, nitric acid, sodium hydroxide, sodiumcarbonate, or ammonia, can be used. In particular, in order to preventdeterioration of surrounding members, preferred are nitric acid andammonia that do not contain impurities, such as alkali metals,alkaline-earth metals, halogen elements, sulfur, phosphorus, and boron.

The lower limit of the content of the copper particles 41 in the ink ispreferably 5% by mass, more preferably 10% by mass, and still morepreferably 20% by mass. The upper limit of the content of the copperparticles 41 in the ink is preferably 50% by mass, more preferably 40%by mass, and still more preferably 30% by mass. When the content of thecopper particles 41 is equal to or more than the lower limit, thecoating film 42 can be made denser. On the other hand, when the contentof the copper particles 41 exceeds the upper limit, there is a concernthat the thickness of the coating film 42 may become non-uniform.

(Other Components)

The ink may contain, in addition to the copper particles 41, adispersing agent. The dispersing agent is not particularly limited, andvarious dispersing agents that are able to disperse the copper particles41 satisfactorily can be used. The lower limit of the molecular weightof the dispersing agent is preferably 2,000. On the other hand, theupper limit of the molecular weight of the dispersing agent ispreferably 30,000. By using a dispersing agent whose molecular weight iswithin the range described above, the copper particles 41 can besatisfactorily dispersed in the ink so that the quality of the coatingfilm 42 can be dense and free from defects. When the molecular weight ofthe dispersing agent is less than the lower limit, there is a concernthat it may not be possible to sufficiently obtain the effect ofpreventing aggregation of the copper particles 41 to maintaindispersion. On the other hand, when the molecular weight of thedispersing agent exceeds the upper limit, there is a concern that thedispersing agent may become excessively bulky and inhibit sintering ofthe copper particles 41, resulting in generation of voids, during firingof the coating film 42. Furthermore, when the dispersing agent isexcessively bulky, there is a concern that the denseness of the coatingfilm 42 may be decreased and that decomposition residues of thedispersing agent may decrease conductivity.

From the viewpoint of preventing deterioration of surrounding members,the dispersing agent preferably does not contain sulfur, phosphorus,boron, halogens, or alkalis. Preferred examples of the dispersing agent,whose molecular weight is within the range described above, includeamine-based polymer dispersing agents, such as polyethyleneimine andpolyvinylpyrrolidone; hydrocarbon-based polymer dispersing agents havingcarboxy groups in their molecules, such as polyacrylic acid andcarboxymethyl cellulose; and polymer dispersing agents having polargroups, such as POVAL (polyvinyl alcohol), styrene-maleic acidcopolymers, olefin-maleic acid copolymers, and copolymers having apolyethyleneimine moiety and a polyethylene oxide moiety in a molecule.

The dispersing agent may be dissolved in water or a water-solubleorganic solvent, and the resultant solution may be added to the ink. Inthe case where the dispersing agent is added to the ink, the lower limitof the content of the dispersing agent is preferably 1 part by massrelative to 100 parts by mass of the copper particles 41. On the otherhand, the upper limit of the content of the dispersing agent ispreferably 60 parts by mass relative to 100 parts by mass of the copperparticles 41. When the content of the dispersing agent is less than thelower limit, there is a concern that the effect of preventingaggregation of the copper particles 41 may become insufficient.Contrarily, when the content of the dispersing agent exceeds the upperlimit, there is a concern that the excessive dispersing agent mayinhibit sintering of the copper particles 41, resulting in generation ofvoids, during firing of the coating film 42. There is also a concernthat decomposition residues of the dispersing agent may remain asimpurities in the sintered body to decrease conductivity.

As the dispersion medium in the ink, for example, water can be used. Inthe case where water is used as the dispersion medium, the lower limitof the water content is preferably 20 parts by mass relative to 100parts by mass of the copper particles 41. On the other hand, the upperlimit of the water content is preferably 1,900 parts by mass relative to100 parts by mass of the copper particles 41. Water, which is thedispersion medium, for example, sufficiently swells the dispersingagent, and satisfactorily disperses the copper particles 41 surroundedby the dispersing agent. However, when the water content is less thanthe lower limit, there is a concern that the effect of swelling thedispersing agent may become insufficient. Contrarily, when the watercontent exceeds the upper limit, the content of the copper particles 41in the ink decreases, and there is a concern that it may not be possibleto form a good sintered body having required thickness and density.

An organic solvent may be optionally added to the ink for the purpose ofviscosity adjustment, vapor pressure adjustment, and the like. Variouswater-soluble organic solvents can be used as such an organic solvent.Specific examples thereof include alcohols, such as methyl alcohol,ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol,isobutyl alcohol, sec-butyl alcohol, and tert-butyl alcohol; ketones,such as acetone and methyl ethyl ketone; esters of polyhydric alcohols,such as ethylene glycol and glycerol, and others; and glycol ethers,such as ethylene glycol monoethyl ether and diethylene glycol monobutylether.

In the case where an organic solvent is added to the ink, the lowerlimit of the content of the organic solvent is preferably 30 parts bymass relative to 100 parts by mass of the copper particles 41. On theother hand, the upper limit of the content of the organic solvent ispreferably 900 parts by mass relative to 100 parts by mass of the copperparticles 41. When the content of the organic solvent is less than thelower limit, there is a concern that the effect of adjusting theviscosity and vapor pressure of the ink may not be obtainedsufficiently. Contrarily, when the content of the organic solventexceeds the upper limit, for example, the effect of swelling thedispersing agent by water may become insufficient, and there is aconcern that aggregation of the copper particles 41 may occur in theink.

Furthermore, in the case where the copper particles 41 are produced by aliquid-phase reduction method, the copper particles 41 precipitated in aliquid-phase (aqueous solution) reaction system are subjected to stepsof separation by filtration, washing, drying, disintegration, and thelike, and the resultant powder can be used to prepare the ink. In thiscase, the copper particles 41 in the form of powder, a dispersionmedium, such as water, and optionally, a dispersing agent, an organicsolvent, and the like are mixed at predetermined proportions to therebyobtain an ink containing the copper particles 41. In this case, theliquid phase (aqueous solution) in which the copper particles 41 areprecipitated is preferably used as a starting material to prepare theink. Specifically, the liquid phase (aqueous solution) containing theprecipitated copper particles 41 is subjected to processes, such asultrafiltration, centrifugal separation, washing with water, andelectrodialysis, to remove impurities, and optionally, is concentratedto remove water. Alternatively, conversely, after water is added toadjust the concentration of the copper particles 41, an organic solventis further added as necessary at a predetermined ratio to prepare an inkcontaining the copper particles 41. This method can prevent generationof coarse and non-uniform particles due to aggregation of the copperparticles 41 during drying and facilitate formation of a dense anduniform sintered body.

As the method of applying the ink in which the copper particles 41 aredispersed to one surface of the base film 2, a known coating method,such as spin coating, spray coating, bar coating, die coating, slitcoating, roll coating, or dip coating, can be used. Furthermore, byusing screen printing, a dispenser, or the like, the ink may be appliedto only portions of one surface of the base film 2. After application ofthe ink, for example, by drying at a temperature equal to or higher thanthe room temperature, a coating film 42 is formed. The upper limit ofthe drying temperature is preferably 100° C. and more preferably 40° C.When the drying temperature exceeds the upper limit, there is a concernthat cracks may occur in the coating film 42 due to sudden drying of thecoating film 42.

(Copper Particle Bond Layer Formation Step)

In the copper particle bond layer formation step, a copper particle bondlayer 3 a is formed by firing the coating film 42.

In the copper particle bond layer formation step, as shown in FIG. 4B,firing sinters the copper particles 41 together and fixes the sinteredbody to one surface of the base film 2 to thereby form a copper particlebond layer 3 a. The dispersing agent and other organic substances thatcan be contained in the ink are volatilized or decomposed by firing.Furthermore, it is considered that metal oxides, specifically, mainlycopper oxide, originating from the copper particles 41 are dispersed bythe firing into a surface layer of the base film 2, and thereby, amodified layer 2 a is formed on the one surface side of the base film 2.

The firing is preferably performed in an atmosphere containing a certainamount of oxygen in order to promote oxidation of the copper particles41 in the vicinity of the interface between the copper particle bondlayer 3 a and the base film 2 so that the surface layer of the base film2 can be surely modified. In this case, the lower limit of the oxygenconcentration in the firing atmosphere is preferably 1 ppm by volume andmore preferably 10 ppm by volume. Furthermore, the upper limit of theoxygen concentration is preferably 10,000 ppm by volume and morepreferably 1,000 ppm by volume. When the oxygen concentration is lessthan the lower limit, the amount of copper oxide generated in thevicinity of the interface between the copper particle bond layer 3 a andthe base film 2 decreases, and there is a concern that the surface layerof the base film 2 may not be sufficiently modified. Contrarily, whenthe oxygen concentration exceeds the upper limit, there is a concernthat the conductivity of the copper particle bond layer 3 a may bedecreased by excessive oxidation of the copper particles 41.

The lower limit of the firing temperature is preferably 250° C., morepreferably 300° C., and still more preferably 330° C. On the other hand,the upper limit of the firing temperature is preferably 500° C. and morepreferably 400° C. When the firing temperature is less than the lowerlimit, the amounts of copper oxide and the like generated in thevicinity of the interface between the copper particle bond layer 3 a andthe base film 2 decrease, and there is a concern that the one surfaceside of the base film 2 may not be sufficiently modified. Contrarily,when the firing temperature exceeds the upper limit, there is a concernthat the base film 2 may become deformed. However, the firingtemperature is not limited as long as it is a temperature at which thesintered body of the copper particles 41 is fixed to the base film 2,and can be set appropriately in combination with the firing time, whichwill be described later, for example, at about 100° C. or lower.

The lower limit of the firing time is preferably 80 minutes and morepreferably 100 minutes. On the other hand, the upper limit of the firingtime is preferably 180 minutes and more preferably 150 minutes. When thefiring time is less than the lower limit, there is a concern that theone surface side of the base film 2 may not be sufficiently modified.Contrarily, when the firing temperature time exceeds the upper limit,there is a concern that the base film 2 may become deformed.Particularly preferably, the firing temperature and the firing time eachare set to be in the range described above. Thereby, the one surfaceside of the base film 2 can be sufficiently modified while preventingdeformation of the base film 2, and the lightness L* and chromaticitiesa* and b* of the one surface of the base film 2 can be easily adjustedto be in desired ranges.

(Patterning Step)

In the patterning step, by patterning the copper particle bond layer 3 aformed in the copper particle bond layer formation step, a conductivepattern is formed on one surface of the base film 2. The printed circuitboard 1 shown in FIG. 1 is obtained by the patterning step. In thepatterning step, patterning can be performed by using a known etchingtechnique.

Next, with reference to FIGS. 4A and 4C, a method for manufacturing theprinted circuit board 11 will be described. The method for manufacturingthe printed circuit board 11 includes a step of forming a coating film42 by applying an ink containing copper particles 41 to one surface of abase film 2, a step of forming a copper particle bond layer 3 a (copperparticle sintered layer) by firing the coating film 42, a step offorming a metal plating layer 12 a on the outer surface of the copperparticle bond layer 3 a, and a step of patterning a laminated bodyconsisting of the copper particle bond layer 3 a and the metal platinglayer 12 a.

The coating film formation step and the copper particle bond layerformation step in the method for manufacturing the printed circuit board11 are the same as the coating film formation step and the copperparticle bond layer formation step for the printed circuit board 1.Furthermore, patterning in the patterning step of the method formanufacturing the printed circuit board 11 can be performed by using aknown etching technique as in the printed circuit board 1. Accordingly,the metal plating layer formation step only will be described below.

(Metal Plating Layer Formation Step)

In the metal plating layer formation step, voids of the copper particlebond layer 3 a are filled with a plating metal and the plating metal isdeposited on one surface of the copper particle bond layer 3 a.

The plating method for forming the metal plating layer 12 a is notparticularly limited, and may be electroless plating or electroplating.It is preferable to use electroless plating by which the voids betweenthe copper particles constituting the copper particle bond layer 3 a canbe more appropriately filled, and the effect of modifying the base film2 is easily improved by performing a heat treatment after filling withthe plating metal.

In the case where the electroless plating is used, its procedure is notparticularly limited. For example, electroless plating may be performedthrough processes, such as a cleaner process, a water-washing process,an acid treatment process, a water-washing process, a pre-dip process,an activator process, a water-washing process, a reduction process, anda water-washing process, and by known methods.

In the case where the electroplating is used, its procedure is notparticularly limited, and, for example, may be appropriately selectedfrom known electroplating baths and plating conditions.

Furthermore, after the voids of the copper particle bond layer 3 a arefilled with a plating metal, a heat treatment is preferably performed.Since the amount of copper oxide in the vicinity of the interfacebetween the copper particle bond layer 3 a and the base film 2 isfurther increased by the heat treatment, modification of the one surfaceof the base film 2 can be further promoted. Furthermore, the heattreatment temperature and the heat treatment time can be the same as thefiring temperature and the firing time in the copper particle bond layerformation step for the printed circuit board 1.

Next, with reference to FIGS. 4A and 4D, a method for manufacturing theprinted circuit board 21 will be described. The method for manufacturingthe printed circuit board 21 includes a step of forming a coating film42 by applying an ink containing copper particles 41 to one surface of abase film 2, a step of forming a copper particle bond layer 3 a (copperparticle sintered layer) by firing the coating film 42, a step offorming a metal plating layer on the outer surface of the copperparticle bond layer 3 a, and a step of patterning a laminated bodyconsisting of the copper particle bond layer 3 a and the metal platinglayer.

The coating film formation step and the copper particle bond layerformation step in the method for manufacturing the printed circuit board21 are the same as the coating film formation step and the copperparticle bond layer formation step for the printed circuit board 1.Furthermore, patterning in the patterning step of the method formanufacturing the printed circuit board 21 can be performed by using aknown etching technique as in the printed circuit board 1. Accordingly,the metal plating layer formation step only will be described below.

(Metal Plating Layer Formation Step)

The metal plating layer formation step includes a step of forming afirst plating layer 12 a, which is the same as the metal plating layer12 a, and a step of forming a second plating layer 22 a on a surface ofthe first plating layer 12 a. The step of forming the first platinglayer is the same as the step of forming the metal plating layer 12 a,and therefore, a description thereof will be omitted.

The plating method for forming the second plating layer 22 a is notparticularly limited, and may be electroless plating or electroplating.It is preferable to use electroplating by which the thickness can beadjusted easily and accurately and the second plating layer 22 a can beformed in a relatively short period of time.

In the case where the electroless plating is used, its procedure is notparticularly limited, and the electroless plating can be performed bythe same procedure as that in the case where the metal plating layer 12a is formed. Furthermore, in the case where the electroplating is used,its procedure is not particularly limited, and the electroplating can beperformed by the same procedure as that in the case where the metalplating layer 12 a is formed.

<Advantages>

In the method for manufacturing a printed circuit board, the printedcircuit board can be easily and reliably manufactured. Furthermore, inthe method for manufacturing a printed circuit board, in the metalplating layer formation step, by performing a heat treatment afterfilling with the plating metal, modification of the one surface side ofthe base film 2 can be promoted, and the lightness L* and chromaticitiesa* and b* of the one surface of the base film 2 can be more reliablyadjusted.

[Other Embodiments]

It should be considered that the embodiments disclosed this time areillustrative and non-restrictive in all aspects. The scope of thepresent invention is not limited to the embodiments described above butis defined by the appended claims, and is intended to include allmodifications within the meaning and scope equivalent to those of theclaims.

For example, the printed circuit board may not necessarily haveflexibility. Furthermore, the printed circuit board does not necessarilyhave a conductive pattern including a copper particle bond layer on onlyone surface of a base film, and the conductive pattern may be disposedon each of both surfaces of the base film. Furthermore, in the casewhere the printed circuit board has the conductive patterns on bothsurfaces of the base film, metal plating layers may be disposed on theouter surfaces of the two copper particle bond layers. In the case wherethe printed circuit board has the conductive patterns on both surfacesof the base film, the both surface sides of the base film can bemodified, and the lightness L* and chromaticities a* and b* of the bothsurfaces can be easily adjusted. Thereby, it is possible to promote theeffect of suppressing the false detection rate for circuit defects.

The base film may not necessarily include a modified layer on onesurface side thereof. In the printed circuit board, for example, thebase film may be formed as a laminated body consisting of a plurality ofresin films, and the lightness L* may be adjusted to be in the rangedescribed above by the film disposed at the outermost surface.

The printed circuit board is not necessarily formed by a subtractivemethod, and may be formed by a semi-additive method.

Furthermore, the present invention is also targeted at an electroniccomponent which uses the printed circuit board. Specifically, in thepresent invention, for example, an electronic component in which theprinted circuit board according to any of the embodiments describedabove is electrically connected to an element, such as a semiconductordevice or chip resistor, is also within the intended scope of theinvention. Since the electronic component includes the printed circuitboard which can reduce the inspection error rate in determination ofcircuit defects, it is possible to reduce the rate of occurrence ofcircuit defects.

EXAMPLES

The present invention will be described more in detail on the basis ofexamples. However, it is to be understood that the present invention isnot limited to the examples.

[Nos. 1 to 3]

Copper particles with a mean particle size of 60 nm, which were obtainedby a liquid-phase reduction method, were dispersed in water serving as asolvent to prepare an ink having a copper concentration of 26% by mass.Next, a polyimide film with an average thickness of 25 μm was used as abase film, and a wet blast treatment and a plasma treatment wereperformed in this order on one surface of the polyimide film(hereinafter, the wet blast treatment and the plasma treatment may becollectively referred to as the “surface treatment”). Furthermore, theink was applied to the one surface of the polyimide film and dried inair to form a coating film. Then, the coating film was fired in anitrogen atmosphere with an oxygen concentration of 100 ppm by volumefor 120 minutes at 350° C. In such a manner, printed circuit board basematerials Nos. 1 to 3, each including a copper particle bond layer(average thickness 150 nm) fixed to the polyimide film, were obtained.

Subsequently, electroless copper plating was performed on one surface ofthe copper particle bond layer of each of the printed circuit board basematerials Nos. 1 to 3 to form a first plating layer with an averagethickness of 1 μm. Furthermore, copper electroplating was performed toform a second plating layer with an average thickness of 25 μm. Theprinted circuit board base materials provided with the first platinglayer and the second plating layer were subjected to a heat treatment ina nitrogen atmosphere with an oxygen concentration of 100 ppm by volumefor 120 minutes at 350° C., and then, by using a subtractive method,printed circuit boards Nos. 1 to 3 were fabricated. Note that a sodiumhydroxide aqueous solution was used as an etchant in the subtractivemethod. Furthermore, when the cross section in the thickness directionof each of the base films Nos. 1 to 3 was observed with a scanningelectron microscope (SEM), it was found that an insulating layer wasformed in a region extending from the surface fixed to the copperparticle bond layer to a depth of 10 nm, the insulating layer having adifferent composition from the other region of the base film. It isconsidered from this result that the region having the differentcomposition is modified (i.e., a modified layer).

[No. 4]

A polyimide film with an average thickness of 25 μm was used as a basefilm, and one surface of the polyimide film was subjected to the samesurface treatment as that for Nos. 1 to 3. Furthermore, a rolled copperfoil (average thickness 12 μm) was disposed on one surface of thepolyimide film by using an adhesive to obtain a printed circuit boardbase material No. 4. The printed circuit board base material wassubjected to a heat treatment in a nitrogen atmosphere with an oxygenconcentration of 100 ppm by volume for 120 minutes at 350° C. Then, byusing a subtractive method, a printed circuit board No. 4 wasfabricated. Note that, a sodium hydroxide aqueous solution was used asan etchant in the subtractive method. Note that, in the printed circuitboard No. 4, there is no copper particle bond layer, and a modifiedlayer such as the one described above is not observed.

[Quality of Printed Circuit Board Base Material]

<Lightness and Chromaticity>

The lightness L* and chromaticities a* and b* of one surface of the basefilm before the surface treatment were measured on the printed circuitboard base materials Nos. 1 to 4. The lightness L* and chromaticities a*and b* were measured, by using a colorimeter (“CR-400” manufactured byKONICA MINOLTA, INC.), in accordance with JIS-Z8781-4 (2013). Themeasurement results of the lightness L* and chromaticities a* and b* areshown in Table 1.

<External Transmittance>

The external transmittance for a wavelength of 500 nm in the base filmbefore the surface treatment was measured on the printed circuit boardbase materials Nos. 1 to 4. The external transmittance was measured byusing a “TLV-304-BP” manufactured by Asahi Spectra Co., Ltd. Themeasurement results of the external transmittance are shown in Table 1.

TABLE 1 Before surface treatment Printed External circuit boardLightness Chromaticity Chromaticity transmittance base material L* a* b*(%) No. 1 69.8 10.7 86.3 28.5 No. 2 70.6 13.3 87.2 14.6 No. 3 70.4 13.387.4 23.2 No. 4 71.2 13.5 87.5 21.2

[Quality of Printed Circuit Board and Evaluation]

<Lightness and Chromaticity>

The lightness L* of one surface of the conductive pattern was measuredon the printed circuit boards Nos. 1 to 4 by the same measurement methodas that described above. Furthermore, the lightness L* andchromaticities a* and b* of the conductive pattern non-formed region ofthe base film were measured on the printed circuit boards Nos. 1 to 4 bythe same measurement method as that described above. The measurementresults are shown in Table 2.

<External Transmittance>

The external transmittance for a wavelength of 500 nm in the conductivepattern non-formed region of the base film was measured on the printedcircuit boards Nos. 1 to 4 by the same measuring device as thatdescribed above. The measurement results are shown in Table 2.

<Difference in Lightness>

The absolute value of a difference obtained by subtracting the lightnessL* of the conductive pattern non-formed region of the base film from thelightness L* of one surface of the conductive pattern (difference inlightness L*) was calculated on the printed circuit boards Nos. 1 to 4.The calculation results are shown in Table 2.

<False Detection Rate for Circuit Defects>

A plurality of each of printed circuit boards Nos. 1 to 4 were prepared,and circuit defects were detected, by using an automated opticalinspection system (AOI), on the plurality of printed circuit boards.Next, 100 printed circuit boards each for Nos. 1 to 4 in which circuitdefects were detected by the AOI were inspected visually regarding theaccurate presence or absence of circuit defects by using an opticalmicroscope. The false detection rate for circuit defects in each of theprinted circuit boards Nos. 1 to 4 was calculated in accordance with theformula: (100−A)/100×100 [%], where A is the number of printed circuitboards in which circuit defects were detected by using the opticalmicroscope. The calculation results are shown in Table 2.

TABLE 2 False Lightness L* External detection One surface ChromaticityChromaticity transmittance rate for Printed of Non- a* b* (%) Differencecircuit circuit conductive formed Non-formed Non-formed Non-formed inlightness defects board pattern region region region region L* (%) No. 172.2 46.6 17.7 35.2 6.7 25.6 2 No. 2 71.6 46.3 19.4 34.4 5.9 25.3 1 No.3 71.4 44.1 19.0 30.0 7.3 27.3 1 No. 4 71.1 62.0 11.8 40.4 16.0 9.1 9

[Evaluation Results]

As is evident from Tables 1 and 2, in the printed circuit boards Nos. 1to 3, the lightness L* of the conductive pattern non-formed region is44.1 to 46.6, which is 60 or less. Furthermore, it is evident that thisis based on the fact that the lightness L* of the conductive patternnon-formed region is lower, by 23 or more, than the lightness L* of thebase film of the printed circuit board base material before the surfacetreatment. In contrast, in the printed circuit board No. 4, thelightness L* of the conductive pattern non-formed region is 62.0, whichis more than 60. Furthermore, as is evident from Table 2, in the printedcircuit boards Nos. 1 to 3, the difference between the lightness L* ofthe one surface of the conductive pattern and the lightness L* of theconductive pattern non-formed region of the base film is 25.3 to 27.3,which is 10 or more. In contrast, in the printed circuit board No. 4,the difference in lightness L* is low at 9.1. Accordingly, it is evidentthat, in the printed circuit boards Nos. 1 to 3, the contrast betweenthe conductive pattern non-formed region of the base film and the onesurface of the conductive pattern is sufficiently obtained, resulting inan improvement in inspection accuracy for circuit defects, compared withthe printed circuit board No. 4.

The invention claimed is:
 1. A printed circuit board comprising: a basefilm having an insulating property; and a conductive pattern disposed onat least one surface of the base film, wherein the base film includes amodified layer on one surface side thereof, on which the conductivepattern is disposed, and a non- modified layer on the other surface sidethereof, a composition of the modified layer of the base film variesfrom a composition of the non-modified layer of the base film, themodified layer of the base film has an average thickness of 5 nm or moreand 100 nm or less, wherein the conductive pattern includes a copperparticle bond layer which has a structure in which a plurality of copperparticles adhere and bond together and which is fixed to the base filmand a lightness L* of a conductive pattern non-formed region of the basefilm is 60 or less.
 2. The printed circuit board according to claim 1,wherein the base film does not substantially contain a pigment.
 3. Theprinted circuit board according to claim 1, wherein a main component ofthe base film is a polyimide.
 4. The printed circuit board according toclaim 1, wherein a chromaticity b* of the conductive pattern non-formedregion of the base film is 60 or less.
 5. The printed circuit hoardaccording to claim 1, wherein the absolute value of a difference betweena lightness L* of one surface of the conductive pattern and thelightness L* of the conductive pattern non-formed region of the basefilm is 10 or more.
 6. The printed circuit board according to claim 1,wherein a copper oxide originating from the copper particles of thecopper particle bond layer is dispersed in the modified layer of thebase film.
 7. An electronic component comprising the printed circuitboard according to claim 1, wherein the printed circuit board isconfigured to be electrically connected to a semiconductor device orchip resistor of the electronic component.